This application claims priority from the commonly-owned utility application filed concurrently with the present application entitled xe2x80x9cThermally Stable Multiplexer/Demultiplexer,xe2x80x9d which is incorporated herein in its entirety.
The present invention is directed towards optical communications, and more particularly toward a structure facilitating alignment of the optical elements of a bulk optical multiplexer/demultiplexer.
At the inception of fiber optic communications, typically a fiber was used to carry a single channel of data at a single wavelength. Dense wavelength division multiplexing (DWDM) enables multiple channels at distinct wavelengths within a given wavelength band to be sent over a single mode fiber, thus greatly expanding the volume of data that can be transmitted per optical fiber. The wavelength of each channel is selected so that channels do not interfere with each other and the transmission losses to the fiber are minimized. While typical DWDM allows up to 40 channels to be simultaneously transmitted by a fiber, there is an ongoing effort to further increase the number of channels transmitted for a given wavelength band by an optical fiber.
DWDM requires two conceptually symmetric devices: a multiplexer and a demultiplexer. A multiplexer takes multiple beams or channels of light, each at a discreet wavelength and from a discreet source and combines the channels into a single multi-channel or multiplexed beam. The input is typically a linear array of waveguides such as a linear array of optical fibers. The output is typically a single waveguide such as an optical fiber. A demultiplexer spatially separates a multiplexed beam into separate channels according to wavelength. Input is typically a single input waveguide or fiber and the output is typically a linear array of waveguides such as optical fibers.
There are a number of different DWDM devices known in the art, including array waveguides (see Li, U.S. Pat. No. 5,706,377), devices using a network of filters and/or fiber Bragg gratings for channel separation (see Pan, U.S. Pat. No. 5,748,350), and a variety of bulk optical DWDM devices. Bulk optical multiplexers and demultiplexers consist of discreet optically aligned optical elements. For example, a wavelength dispersive element such as a reflective diffraction grating, a focusing optic such as a lens, and a waveguide array which may consist of a multi-channel or multiplex waveguide such as a single mode optical fiber and a linear array of single channel waveguides, typically also single mode optical fibers. In a demultiplexing operation, the multi-channel or multiplexed optical signal is emitted from the multi-channel waveguide, directed through and collimated by the focusing optic, and reflected off the diffraction grating. The diffraction grating divides the multi-channel beam into single channel beam components which are reflected through the focusing optic and focused by the focusing optic to optical focal points coupling with the single channel optical waveguides. The multiplexer simply works in reverse, with single channel signals being emitted from the single channel optical fibers, combined into a multiplex signal and coupled to the multiplex optical fiber. Because a single device can perform as a multiplexer or a demultiplexer, it is referred to as a (de)multiplexer herein. Critical to the proper operation of a bulk optic (de)multiplexer is maintaining proper optical alignment of the waveguide array, focusing optic, and diffraction grating to provide efficient coupling of the optical signals to the respective waveguides with minimal or no crosstalk. To date, providing a structure for facilitating proper alignment of the optical elements and for maintaining the optical elements in the desired optical alignment has proven illusive.
Schultheiss, U.S. Pat. No. 4,718,056, is directed to a bulk optical (de)multiplexer including a diffraction grating, a lens and an optical fiber harness. In Schultheiss, the diffraction grating, lens, and fiber harness are all mounted to a frame by adjustable mounts. While having each of the optical elements on its own adjustable mount clearly makes it possible to optimize the optical alignment of the (de)multiplexer optical elements, it actually over complicates alignment because none of the optical elements are fixed relative to the frame to provide a reference point, thus necessitating adjustment of each element during optical alignment.
Ignatuc, U.S. Pat. No. 5,195,707, is directed to an optic positioning device for holding an optical element which has a center and for adjusting the optical element relative to the center. The positioning device includes a supporting base having a concave spherical surface and a holding body having a convex spherical surface which is slidably mated with the concave spherical surface. Both the concave and convex spherical surfaces have radial centers at the center of the optical element. Ignatuc allows for gimbaled movement of the optical element about its optical center. However, the mating concave and convex spherical surfaces provide a large surface contact area which can make it difficult to make small, precise movements of the optical element due to xe2x80x9csticktionxe2x80x9d between the surfaces. Ignatuc also requires that both spherical surfaces be made to precise tolerances in order to insure the center of the optical element remains at a fixed location. This increases manufacturing costs.
The present invention is intended for overcoming one or more of the problems discussed above.
A first aspect of the invention is a bulk optic (de)multiplexer for fiber optic communications systems including a diffraction grating having a diffraction surface, a waveguide array including a plurality of waveguides having an input/output end for emitting and receiving optical signals, and a focusing optic in optical communication between the diffraction grating and the waveguide array along an optical axis. The focusing optic focuses beams from the diffraction surface of the grating for optical coupling with the input/output ends of the waveguides. The (de)multiplexer further includes a frame. A fixed mount is provided between the focusing optic and the frame. A first adjustable mount is provided between the waveguide array and the frame and a second adjustable mount is provided between the diffraction grating and the frame. Preferably, the optical axis corresponds to a Z axis of orthogonal X, Y, Z axes and the waveguides of the waveguide array are aligned with the input/output ends along an input/output axis. The first adjustable mount is configured to provide for linear movement of the waveguide array along the Z axis and independent movement of the input/output axis within a plane parallel to the X, Y axes. The second adjustable mount is preferably configured to provide only for gimbaled movement of the grating about a point on the diffraction surface of the grating intersecting the optical axis.
A second aspect of the present invention is an attachment assembly for attaching a diffraction grating having a diffraction surface of an optical (de)multiplexer to a frame of the (de)multiplexer, the (de)multiplexer having optical elements in addition to the grating, the optical elements being attached to the frame and aligned along an optical axis. The attachment assembly includes a grating mount having a leading surface to which the grating is attached with a diffraction surface in a select orientation relative to the grating mount and a spherical surface having a radial center at a point on the diffraction surface of the grating. A receptacle on the frame has a surface which is conical about a central axis receiving the spherical surface of the grating mount with the optical axis intersecting the point on the refractive surface of the grating. A clamp or stay is operatively associated with the grating mount and the frame for fixing the grating mount relative to the frame with the diffraction surface in a select orientation relative to the optical axis.
A third aspect of the present invention is a method of optically aligning a (de)multiplexer for fiber optic communications systems. The (de)multiplexer includes a diffraction grating having a diffraction surface for dividing a multi-channel incident beam into single channel beams, a waveguide array including a plurality of waveguides having an input/output end for receiving the single channel beams, and a focusing optic in optical communication between the diffraction grating and the waveguide array along an optical axis, the focusing optic focusing the single channel beams from the diffraction surface of the grating for optical coupling with the input/output ends of the waveguides. The method includes fixing the focusing optic to a frame of the (de)multiplexer to define an optical axis of the (de)multiplexer corresponding to the optical axis of the focusing optic, moving the waveguide array relative to the focusing optic linearly along the optical axis and moving the grating only by rotating the grating about three orthogonal axes at a point on the grating surface intersected by the optical axis.
The bulk optic (de)multiplexer of the present invention provides a combination of fixed and adjustable mounts which eases alignment of the (de)multiplexer optical elements. The fixed mount of the focusing lens allows the optical axis of the focusing lens to define a reference about which the other optical elements can be aligned. The adjustable mount between the grating and the frame not only provides for gimbaled movement of the grating about a point on the grating surface intersected by the optical axis to aid in proper alignment of the grating, it also provides a structure which prevents a change of orientation of the grating relative to the optical axis while providing movement of the grating only along the optical axis due to temperature changes or clamping of the adjustable mount. The adjustable mount associated with the waveguide array allows for independent movement of the waveguide array along the optical axis and for independent movement of the waveguide array in a plane normal to the optical axis. This independence of movement further facilitates efficient alignment. The apparatus further facilitates the claimed method of aligning the (de)multiplexer which simplifies and expediates alignment of the optical elements.